The invention relates to new and useful improvements to a shielded mounting rack for electrical printed circuit board assemblies. More particularly, the invention relates to a mounting rack for electrical printed circuit boards whose basic design as a rule includes two side wall plates that are connected to one another by transverse rails. The front and rear of the mounting rack are each formed by a pair of upper and lower transverse rails. Mutually opposite pairs of guide rails are latched into these transverse rails to form so-called installation slots. The printed circuit boards, also called printed circuit board assemblies, can be inserted into these slots, parallel to one another and each oriented vertically.
As a rule, the printed circuit boards have a so-called front panel which covers a front face of the printed circuit board and is used as the retaining surface for a wide range of display, control and contact elements. The front panels of printed circuit boards that have been pushed vertically into a mounting rack next to one another then form a virtually continuous surface, covering the front of the mounting rack.
To achieve electromagnetic shielding, printed circuit board front panels are generally manufactured from an electrically conductive material, or are at least covered by such a material.
German Laid Open Publication DE 36 04 860 C2 and U.S. Pat. No. 4,631,641 both disclose arrangements for radio-frequency screening of front panels located vertically next to one another on the front of a mounting rack. These front panels are electrically conductively connected to one another via vertical spring contact strips. This allows the vertical grooves on the front of a mounting rack to be effectively shielded against the ingress and emission of electromagnetic interference.
One problem associated with such electromagnetic shielding arises at the lower and upper narrow sides of each front panel. As a rule, there are admittedly generally mounting screws at these points, so that the front panels can be screwed, for example, to rails with threaded holes in them. These rails are inserted into transversely running retaining grooves on the fronts of the lower and upper transverse rails of the mounting rack. Furthermore, these mounting screws allow an electrically conductive connection to be made between the ends of each of the front panels and the mounting rack frame, via its transverse rails. However, these are only point contacts and do not extend over the entire width of the associated front panels. Particularly if the printed circuit board assemblies have broad front panels and occupy a volume of more than one standard unit inside the mounting rack, such an electrically conductive point contact between the front panel and the mounting rack transverse rails running behind its ends is inadequate for reliable electromagnetic shielding.
The same problem also arises with covering panels, for example on the rear of mounting racks. Once again, these generally make only point contact via screws with the transverse rails extending along the top and bottom of the rear of the mounting rack.
In this context, German Patent DE 195 23 257 C1, which was published after the original filing date of the present application, discloses a mounting rack in which the metallic or metallized front panels of the assemblies are conductively connected to the front profiled rails by means of leaf springs. Each leaf spring has a fixing part which is attached to a limb (forming a groove) of the profiled rail. The leaf springs themselves have resilient parts, thus allowing electrical contact to be made between the front panel and the profiled rail upon insertion.
German Utility Model DE 296 18 625 U1 discloses another shielded mounting rack. The module rails of this rack each have an upper and a lower engagement groove located behind the rail's bearing surface. Spring contact strips are provided which can be snapped onto the bearing surfaces and latch elastically into the engagement grooves, thereby engaging behind the bearing surfaces. The spring contact strips have a number of punch-outs which point toward the front of the mounting rack and are resilient, as well as a number of contact claws pointing toward the bearing surface.
The spring contact strips on conventional shielded mounting racks cannot completely overcome the problem of damage potentially caused by wear, particularly if pressure beyond that from the contact with the front panels is exerted on the contact springs of the spring strips. Gripping levers are sometimes fitted to the longitudinal ends of the front panels of the assemblies, to allow the associated assembly to be levered out of the mounting rack. These levers in particular exert considerable point bending forces on the spring contact strips.